1. Field of the Invention
The present invention relates to a tool damage/abnormality detecting device for detecting damage/abnormality, such as breakage of a tool used in machining, in machining workpieces of the same material into one and the same shape or continuously machining a large number of workpieces, such as components of the same specifications, by using a machine tool, and more specifically, to a technique to enable the detecting device to appropriately easily set an allowable range for indexes used for tool damage/abnormality detection. Typical tools to be subjected to damage/abnormality detection include a drill of a machining center, a tapping tool for tapping work, etc.
2. Description of the Related Art
In general, a tool used in a machine tool has its edge worn so that its cutting resistance gradually increases with the passage of machining time. With the passage of additional time, moreover, breakage of the tool, as well as its abrasion, results in damage or other similar abnormality, so that necessary machining accuracy for workpieces cannot be maintained. In such a case, it is naturally inappropriate to continue the same machining, and the tool must be replaced with a new one. The aforesaid damage or other similar abnormality will hereinafter be referred to as damage/abnormality.
Already proposed on this background are techniques for automatically detecting a situation equivalent to tool damage/abnormality in the currently prevailing unattended machining or long-time automatic machining. Many of the conventionally proposed techniques are based on a system described in JP 3446518B and JP2003-326438A and JP2004-130407A. In this system, indexes indicative of loads that act on a tool for machining a workpiece (torque of a motor for driving the tool and thrust load applied to the tool) are detected in a plurality of machining cycles. For each machining cycle, the index for the present machining cycle and an average of the indexes for the machining cycles preceding the present machining cycle are compared. If the result of the comparison is deviated from an allowable range of preset indexes, it is concluded that the tool is subject to damage/abnormality.
In setting the allowable range for an index, threshold values (normally, upper and lower limit values) for the index are preset, so that it is important how to determine the preset threshold values. If the preset threshold values are inappropriate, detection of damage/abnormality may be delayed to cause machining failure (because the allowable range is upwardly or downwardly too wide). If the detection of damage/abnormality is premature (or if the allowable range is upwardly or downwardly too narrow), on the other hand, tools with sufficient life expectancy may be wasted.
Actually, so far, no satisfactory technology development has been made to provide a method for appropriately setting threshold values for indexes indicative of loads that act on tools.
There is a method, for example, in which threshold values for an index (load average) are determined by multiplying, for example, a load average obtained by a cutting test or a load average obtained immediately before machining by a given coefficient. However, there is no designation of a specific method for setting the given coefficient. According to JP 3446518B in which “abnormality detecting method for rotary tool and apparatus therefor” is described, by way of example, threshold values are set in accordance with the changing rate of a load value. By doing this, abnormality can be detected accurately and appropriately on a real-time basis. More specifically, the threshold values are set by multiplying a damage load level Lmax fixed according to the size of a drill or the like by a constant k within the range of 0 to 1 that is settled depending on a changing rate ΔFn of the load value.
However, there is no definite indication of specific methods for determining the aforesaid values Lmax and k. In general, data on damage loads vary depending on the machine, tool, work material, machining conditions, etc., so that it is very desirable to collect the data on the site of work. In the case of a drill or tap, although its abnormality can be detected during a machining cycle, it can be dealt with only after the termination of the machining cycle. Thus, real-time detection is not essential.